A near-Earth object (NEO) is a small Solar System body whose orbit brings it into proximity with Earth. All NEOs have a closest approach to the Sun (perihelion) of less than 1.3 astronomical unit (AU).[2] They include about thirteen thousand near-Earth asteroids (NEAs), more than one hundred near-Earth comets (NECs), and a number of solar-orbiting spacecraft and meteoroids, large enough to be tracked in space before striking the Earth. It is now widely accepted that collisions in the past have had a significant role in shaping the geological and biological history of the planet.[3] NEOs have become of increased interest since the 1980s because of increased awareness of the potential danger some of the asteroids or comets pose to Earth, and active mitigations are being researched.[4]

NEAs have orbits that lie partly between 0.983 and 1.3 AU away from the Sun.[5] When an NEA is detected it is submitted to the IAU's Minor Planet Center for cataloging. Some NEAs‍ '​ orbits intersect that of Earth's so they pose a collision danger.[6] The United States, European Union, and other nations are currently scanning for NEOs[7] in an effort called Spaceguard.

In the United States, NASA has a congressional mandate to catalogue all NEOs that are at least 1 kilometer wide, as the impact of such an object would be catastrophic. As of June 2015[update], there have been 872 NEAs larger than 1 km discovered, of which 153 are potentially hazardous.[8] It was estimated in 2006 that 20% of the mandated objects have not yet been found.[7] As a result of NEOWISE in 2011, it is estimated that 93% of the NEAs larger than 1 km have been found and that only about 70 remain to be discovered.[9] Our inventory is much less complete for smaller objects, which still have potential for large scale damage.

Potentially hazardous objects (PHOs) are currently defined based on parameters that measure the object's potential to make threatening close approaches to the Earth.[10] Mostly objects with an Earth minimum orbit intersection distance (MOID) of 0.05 AU or less and an absolute magnitude (H) of 22.0 or brighter (a rough indicator of large size) are considered PHOs. Objects that cannot approach closer to the Earth (i.e. MOID) than 0.05 AU (7,500,000 km; 4,600,000 mi), or are smaller than about 150 m (500 ft) in diameter (i.e. H = 22.0 with assumed albedo of 13%), are not considered PHOs.[2] The NASA Near Earth Object Catalog also includes the approach distances of asteroids and comets measured in lunar distances,[11] and this usage has become a common unit of measure used by the news media in discussing these objects.

Some NEOs are of high interest because they can be physically explored with lower mission velocity even than the Moon, due to their combination of low velocity with respect to Earth (ΔV) and small gravity, so they may present interesting scientific opportunities both for direct geochemical and astronomical investigation, and as potentially economical sources of extraterrestrial materials for human exploitation.[12] This makes them an attractive target for exploration.[13] As of 2012, three near-Earth objects have been visited by spacecraft: 433 Eros, by NASA's Near Earth Asteroid Rendezvous probe,[14]25143 Itokawa, by the JAXAHayabusa mission,[15] and 4179 Toutatis, by CNSA'sChang'e 2 spacecraft.[4][16]

Human perception of near-Earth objects as benign objects of fascination or killer objects with high risk to human society have ebbed and flowed in the short period of human history that NEOS have been scientifically observed.[17]

More recently, a typical frame of reference for looking at NEOs has been through the scientific concept of risk. In this frame, the risk that any near-Earth object poses is typically seen through a lens that is a function of both the culture and the technology of human society. "NEOs have been understood differently throughout history." Each time an NEO is observed, "a different risk was posed, and throughout time that risk perception has evolved. It is not just a matter of scientific knowledge."[18]

The annual background frequency used in the Palermo scale for impacts of energy greater than Emegatonnes is estimated as:[19]

For instance, this formula implies that the expected value of the time from now until the next impact greater than 1 megatonne is 33 years, and that when it occurs, there is a 50% chance that it will be above 2.4 megatonnes. This formula is only valid over a certain range of E.

However, another paper[20] published in 2002 – the same year as the paper on which the Palermo scale is based – found a power law with different constants:

This formula gives considerably lower rates for a given E. For instance, it gives the rate for bolides of 10 megatonnes or more (like the Tunguska explosion) as 1 per thousand years, rather than 1 per 210 years as in the Palermo formula. However, the authors give a rather large uncertainty (once in 400 to 1800 years for 10 megatonnes), due in part to uncertainties in determining the energies of the atmospheric impacts that they used in their determination.

On 24 December 2004, minor planet 99942 Apophis (at the time known by its provisional designation 2004 MN4) was assigned a 4 on the Torino scale, the highest rating ever achieved. There was a 2.7% chance of Earth impact on 13 April 2029. However, on 28 December 2004, the risk of impact dropped to zero for 2029, but future potential impact solutions were still rated 1 on the Torino scale. The 2036 risk was lowered to a Torino rating of 0 in August 2006. Apophis has no chance of impacting Earth before 2060.[21]

Previously there was one NEO with a Palermo scale value greater than zero which was (29075) 1950 DA, but after further measurements it was downgraded to -1.81.

NASA maintains a continuously updated Sentry Risk Table of the most significant NEO threats in the next 100 years.[21] All or nearly all of the objects are highly likely to eventually drop off the list as more observations come in, reducing the uncertainties and enabling more accurate orbital predictions. (The list does not include (29075) 1950 DA, because that will not strike for at least 800 years.)[4][22]

Goldstone radar-image asteroid to be studied and then have samples returned to Earth by NASA's OSIRIS-REx spacecraft (launch planned for 2016)

In a 2013 article in Wired Science, David Portree provides an overview of NEO science and proposed asteroidal missions, with an emphasis on the outcome of two conferences held in the 1970s. The International Astronomical Union minor planets workshop was held in Tucson, Arizona in March 1971 and a consensus "emerged that launching spacecraft to asteroids would be 'premature'."[17] "In January 1978, NASA’s Office of Space Science held a workshop at the University of Chicago to "assess the state of asteroid studies and consider options for the future."[17]

Of all of the near-Earth asteroids (NEA) that had been discovered by mid-1977, it was estimated that spacecraft could rendezvous with and return from only about one in 10 using less propulsive energy than is necessary to reach Mars. "Because even the most massive NEA—35 kilometres (22 mi)-wide 1036 Ganymed, discovered in 1924, has a very low surface gravity—landing and takeoff would need very little energy. This meant that a single spacecraft could sample multiple sites on any given NEA."[17] Overall, it was estimated that about one percent of all NEAs might provide opportunities for human-crewed missions, or no more than about ten known NEAs. Therefore, unless the NEA discovery rate were "immediately increased five-fold, no opportunity to launch 'astronaut-scientists' to an NEA was likely to occur within a decade of the Chicago workshop."[17]

Near-Earth asteroids smaller than ~1 meter are near-Earth meteoroids and are listed as asteroids on most asteroid tables. The smallest known near-Earth meteoroid is 2008 TS26 with an absolute magnitude of 33[25] and estimated size of only 1 meter.[26]

There are significantly fewer near-Earth asteroids in the mid-size range than previously thought.

These are objects in a near-Earth orbit without the tail or coma of a comet. As of June 2015[update], 12,745 near-Earth asteroids are known,[8] ranging in size from 1 meter up to ~32 kilometers (1036 Ganymed). The number of near-Earth asteroids over one kilometer in diameter is estimated to be about 981, of which over 90% have been discovered.[9][27][28] There are about 1 million near-Earth asteroids about 40 meters in diameter—of which about 1 percent have been discovered.[29] The composition of near-Earth asteroids is comparable to that of asteroids from the asteroid belt, reflecting a variety of asteroid spectral types.[30]

NEAs survive in their orbits for just a few million years.[5] They are eventually eliminated by planetary perturbations, causing ejection from the Solar System or a collision with the Sun or a planet. With orbital lifetimes short compared to the age of the Solar System, new asteroids must be constantly moved into near-Earth orbits to explain the observed asteroids. The accepted origin of these asteroids is that asteroid-belt asteroids are moved into the inner Solar System through orbital resonances with Jupiter. The interaction with Jupiter through the resonance perturbs the asteroid's orbit and it comes into the inner Solar System. The asteroid belt has gaps, known as Kirkwood gaps, where these resonances occur as the asteroids in these resonances have been moved onto other orbits. New asteroids migrate into these resonances, due to the Yarkovsky effect that provides a continuing supply of near-Earth asteroids.[31] The known asteroid with the greatest known chance of impacting Earth is 2010 RF12 with a 1 in 16 chance of impacting Earth on 5 September 2095. Its 7-meter estimated diameter ensures that an impact would cause little damage.

A small number of NEOs are extinct comets that have lost their volatile surface materials, although having a faint or intermittent comet-like tail does not necessarily result in a classification as a near-Earth comet, making the boundaries somewhat fuzzy. The rest of the near-Earth asteroids are driven out of the asteroid belt by gravitational interactions with Jupiter.[5][32]

(Note: Some authors define the Atens group differently: they define it as being all the asteroids with a semi-major axis of less than 1 AU. That is, they consider the Atiras to be part of the Atens. Historically, until 1998, there were no known or suspected Atiras, so the distinction wasn't necessary.)

Atiras and Amors do not cross the Earth's orbit and are not immediate impact threats, but their orbits may change to become Earth-crossing orbits in the future.

As of February 2015[update], 96 near-Earth comets have been discovered.[8] Although no impact of a comet in Earth's history has been conclusively confirmed, the Tunguska event may have been caused by a fragment of Comet Encke.[33] Cometary fragmenting may also be responsible for some impacts from near-Earth objects. It is rare for a comet to pass within 0.1 AU (15,000,000 km; 9,300,000 mi) of Earth.[34]

These near-Earth objects were probably derived from the Kuiper belt, beyond the orbit of Neptune.

Stony asteroids with a diameter of 4 meters (13 ft) impact Earth approximately once per year.[35] Asteroids with a diameter of roughly 7 meters enter Earth's atmosphere with as much energy as Little Boy (the atomic bomb dropped on Hiroshima, approximately 15 kilotonnes of TNT) about every 5 years.[35] These ordinarily explode in the upper atmosphere, and most or all of the solids are vaporized.[36] Every 2,000–3,000 years, objects produce explosions of 10 megatons comparable to the one observed at Tunguska in 1908.[37] Objects with a diameter of one kilometer hit the Earth an average of twice every million year interval.[5] Large collisions with five kilometer objects happen approximately once every twenty million years.[35]

Assuming that these rates will continue for the next billion years, there exist at least 2,000 objects of diameter greater than 1 km that will eventually hit Earth. However, most of these are not yet considered potentially hazardous objects because they are currently orbiting between Mars and Jupiter. Eventually they will change orbits and become NEOs. Objects spend on average a few million years as NEOs before hitting the Sun, being ejected from the Solar System, or (for a small proportion) hitting a planet.[5]

On August 10, 1972, a meteor that became known as 1972 Great Daylight Fireball was witnessed by many people moving north over the Rocky Mountains from the U.S. Southwest to Canada. It was an Earth-grazing meteoroid that passed within 57 kilometers (about 34 miles) of the Earth's surface. It was filmed by a tourist at the Grand Teton National Park in Wyoming with an 8-millimeter color movie camera.[38]

On March 23, 1989, the 300-meter (1,000-foot) diameter Apollo asteroid 4581 Asclepius (1989 FC) missed the Earth by 700,000 kilometers (430,000 mi) passing through the exact position where the Earth was only 6 hours before. If the asteroid had impacted it would have created the largest explosion in recorded history, 12 times as powerful as the Tsar Bomba, the most powerful nuclear bomb ever exploded. It attracted widespread attention as early calculations had its passage being as close as 64,000 km (40,000 mi) from the Earth, with large uncertainties that allowed for the possibility of it striking the Earth.[39]

On 13 October 1990 an Earth-grazing meteoroid EN131090 was observed above Czechoslovakia and Poland. It was moving with a speed of 41.74 km/s (25.94 mi/s) along a 409 km (254 mi) trajectory from the south to the north. The closest approach to the Earth was 98.67 km (61.31 mi). It was captured by two all-sky cameras of the European Fireball Network, which for the first time enabled geometric calculations of the orbit of such a body.[40]

On March 18, 2004, LINEAR announced a 30-meter asteroid, 2004 FH, which would pass the Earth that day at only 42,600 km (26,500 mi), about one-tenth the distance to the Moon, and the closest miss ever noticed. They estimated that similar-sized asteroids come as close about every two years.[41]

On March 31, 2004, two weeks after 2004 FH, meteoroid2004 FU162 set a new record for closest recorded approach, passing Earth only 6,500 km (4,000 mi) away (about one-sixtieth of the distance to the Moon). Because it was very small (6 meters/20 feet), FU162 was detected only hours before its closest approach. If it had collided with Earth, it probably would have harmlessly disintegrated in the atmosphere.

On March 2, 2009, near-Earth asteroid 2009 DD45 flew by Earth at about 13:40 UT. The estimated distance from Earth was 72,000 km (45,000 mi), approximately twice the height of a geostationary communications satellite. The estimated size of the space rock was about 35 meters (115 feet) wide.[42]

On January 13, 2010, at 12:46 UT, near-Earth asteroid 2010 AL30[43] passed at about 122,000 km (76,000 mi). It was approximately 10–15 m (33–49 ft) wide. If 2010 AL30 had entered the Earth's atmosphere, it would have created an air burst equivalent to between 50 kt and 100 kt (kilotons of TNT). The Hiroshima "Little Boy" atom bomb had a yield between 13-18 kt.[44]

On June 28, 2011, an asteroid designated 2011 MD, estimated at 5–20 m (16–66 ft) in diameter, passed within 20,000 km (12,000 mi) of the Earth, passing over the Atlantic Ocean.[45]

On February 15, 2013, 367943 Duende (2012 DA14) passed approximately 27,700 km (17,200 mi) above the surface of Earth. This was closer than satellites in geosynchronous orbit. The asteroid was not visible to the unaided eye.

Although there have been a few false alarms, a number of objects have been known to be threats to the Earth. (89959) 2002 NT7 was the first asteroid with a positive rating on the Palermo Technical Impact Hazard Scale, with approximately one in a million on a potential impact date of approximately February 1, 2019; it is now known that 2002 NT7 will actually safely pass 0.4078 AU (61,010,000 km; 37,910,000 mi) from the Earth on January 13, 2019.[47]

Asteroid (29075) 1950 DA was lost after its discovery in 1950, since its observations over just 17 days were insufficient to determine its orbit, and then rediscovered on December 31, 2000. It has a diameter of about a kilometer (0.6 miles). The chance that it could impact Earth during its March 16, 2880 close approach had been estimated as 1 in 300. This is roughly 50% greater than the combined chance of impact for all other similarly large objects until 2880.[48] The next radar opportunity for 1950 DA is in 2032,[49] and will pinpoint our knowledge of the orbit, but additional optical position measurements have already reduced the probability of a 2880 impact to 1 in 20 000.

^ abcdePortree, David S. (2013-03-23). "Earth-Approaching Asteroids as Targets for Exploration (1978)". Wired. Retrieved 2013-03-27. People in the early 21st century have been encouraged to see asteroids as the interplanetary equivalent of sea monsters. We often hear talk of “killer asteroids,” when in fact there exists no conclusive evidence that any asteroid has killed anyone in all of human history. ... In the 1970s, asteroids had yet to gain their present fearsome reputation ... most astronomers and planetary scientists who made a career of studying asteroids rightfully saw them as sources of fascination, not of worry.